1. Field of the Invention
This invention relates to a method of, and system for, handling multiple backup processes. In one embodiment, the invention provides cascaded multiple target flashcopy consistency group start optimisation for background copy processes.
2. Description of the Related Art
The storage of data in large organisations is of fundamental importance, both for reliability of the data and for the ability to recover data in the event of any hardware failure. Storage area network (SAN) is an architecture that is used when very large amounts of data are needed to be stored in a reliable and secure manner. This technology allows networks to be created that support the attachment of remote computer storage devices such as disk arrays to servers in such a way that, to the operating system, the devices appear as locally attached. It is common in these networks to include a large amount of redundancy, both in the data storage and in the hardware connections between the individual components.
Various methods exist for creating data redundancy. For example, a function such as a flashcopy function enables an administrator to make point-in-time, full volume copies of data, with the copies immediately available for read or write access. The flashcopy can be used with standard backup tools that are available in the environment to create backup copies on tape. A flashcopy function creates a copy of a source volume on a target volume. This copy, as mentioned above, is called a point-in-time copy. When a flashcopy operation is initiated, a relationship is created between a source volume and target volume. This relationship is a “mapping” of the source volume and the target volume. This mapping allows a point-in-time copy of that source volume to be copied to the associated target volume. The relationship exists between this volume pair from the time that the flashcopy operation is initiated until the storage unit copies all data from the source volume to the target volume, or the relationship is deleted.
When the data is physically copied, a background process copies tracks from the source volume to the target volume. The amount of time that it takes to complete the background copy depends on various criteria, such as the amount of data being copied, the number of background copy processes that are running and any other activities that are presently occurring. The flashcopy function works in that the data which is being copied does not actually need to be copied instantaneously, it only needs to be copied just prior to an update causing on overwrite of any old data on the source volume. So, as data changes on the source volume, the original data is copied to the target volume before being overwritten on the source volume.
Therefore, a flashcopy is a feature supported on various storage devices that allows a user or an automated process to make nearly instantaneous copies of entire logical volumes of data. A copy of a source disk is made on a target disk. The copies are immediately available for both read and write access. A common feature of flashcopy like implementations is the ability to reverse the copy. That is, to populate the source disk of a flashcopy map with the contents of the target disk. It is also possible to use flashcopy in cascaded implementations, in which a target disk later becomes the source disk for a further flashcopy or vice versa.
A cascaded configuration of storage volumes is described in detail in U.S. Pat. No. 7,386,695. It is also possible to create multiple cascades of storage volumes which are interlocking at a logical level. A first cascade may comprise storage volumes A, B, C and D which are arranged in a cascade as follows: ABCD, while at a later time a new backup of A may be started that ultimately leads to the creation of AEF. Many different combinations of flashcopy functions and reversed functions are possible, potentially creating complicated multiple cascading storage volumes.
A typical use case for multiple target flashcopy is to create a number of application consistent snapshots of a production disk (the source volume of the flashcopies). This is done using some form of a consistency group in order to guarantee that all the copies are identical. Both the cascaded and the “classical” implementations of multiple target flashcopy do not minimise the read access redirected to the production disk effectively as part of the background copy process used to create the independent copies.
One solution to this problem, which is appropriate to both the classical and cascaded approaches to multiple flashcopy implementations would be to perform a single read of the source disk and multiple writes to each of the targets. However such a solution is not scalable with the number of targets created. This solution also makes it difficult to have different background copy rates for different flashcopy maps. Another solution appropriate to the cascaded approach is to ensure that the last started map completes its background copy process before the background process of the next last started map begins. The problem with this solution is that the length of time taken to produce an independent copy increases with the number of copies required. It is therefore an object of the invention to improve upon the known art.